This invention relates to ethylene polymer compositions having enhanced mechanical properties such as tensile strength and able to maintain these enhanced properties when exposed to high temperatures. Such ethylene polymer compositions are particularly useful as insulation for conductors of electricity.
The outstanding mechanical and electrical properties of ethylene polymers are well known. Ethylene polymers are excellent insulators. They are tough and pliable, and remain so over a wide range of temperatures that encompass both cold and warm conditions found in actual use. They resist attack by water, acids, bases, oils and greases, as well as microorganisms.
The tendency to degrade under thermal-oxidation conditions that ethylene polymers have in common with most organic materials can be controlled by the use in ethylene polymers of stabilizing additives, sometimes termed antioxidants, in modest amounts. The pioneer disclosures of antioxidants for ethylene polymers include the disclosures of phenolic compounds by W. Happoldt in U.S. Pat. No. 2,448,799 of Sept. 7, 1948; thiodipropionate esters by M. Gribbins in U.S. Pat. No. 2,519,755 of Aug. 22, 1950; organic phosphites by A. Hecker in U.S. Pat. No. 2,860,115 of Nov. 11, 1958; organotin mercaptides by W. Leistner in U.S. Pat. No. 3,015,644 of Jan. 2, 1962; and combinations of sulfur compounds such as thioethers and disulfides with carbon black and/or high molecular weight phenolic compounds by W. Hawkins in U.S. Pat. No. 2,889,306 of June 2, 1959. Instead of itemizing the many subsequent disclosures of stabilizing additives for ethylene polymers whose number is staggeringly large, reference is made to the review by L. Nass in "Encyclopedia of Polymer Science and Technology" (N. Bikales, executive editor; J. Wiley-Interscience Publishers, New York), Volume 12 (1970), pages 728 to 737. Nass lists chemicals used on a commercial scale and classifies these and others by the chemical way in which they are believed to function. Nass writes:
"At the moment, there are five methods which have been employed, usually in combination with each other, to inhibit degradation of these polymers (i.e. polyolefin resins).
(1) The use of so-called "primary antioxidants," usually hindered phenols or alkylarylamines, which function mainly by trapping free radicals or by functioning as labile hydrogen donors, and thus interrupting the propagation reactions. PA1 (2) The use of so-called "secondary antioxidants," which consist of organosulfur compounds for the most part, eg, sulfides or thioethers, disulfides, mercaptans, sulfoxylates. These act as peroxide decomposers, combining with hydroperoxides to render them inactive. PA1 (3) The inclusion of inhibitors of color formation, such as tertiary phosphites or phosphonates. These are believed to react preferentially with the oxidized residue of the "primary" antioxidant, thus discharging the color of the typical quinoid bodies. They may also function as peroxide decomposers. PA1 (4) The use of chelating agents or "deactivators" to trap and inactivate trace metal cations since the presence of metallic impurities is considered objectionable. Heavy-metal cations are believed to function as initiators or catalysts for the homolytic cleavage of peroxides into propagating radicals. PA1 (5) The use of ultraviolet-radiation absorbers or other radiation screens or filters to suppress the formation of photoinitiated free radicals." PA1 R.sub.1 and R.sub.2 are the same or different linear or branched alkyl groups having preferably 1 to 6 C atoms, PA1 R.sub.3 is a linear, branched or cyclic alkyl sulfur oxygen interrupted alkyl, phenyl, benzyl or phenyl having alkyls of 1 to 9 carbons (if n=1) or an alkylene group (if n=2), alktriyl (if n=3) and alktetrayl (if n=4), containing 1-20 C atoms, whereby said groups may be substituted for n=1, --C.sub.2 H.sub.4 -- for n=2, ##STR3## for n=4 and X is a linear or branched lower alkylene group. PA1 R.sub.a is --CH.sub.2 (CH.sub.2).sub.n COOR' or alkyl PA1 n=1 to 5 PA1 m=1 to 16 PA1 R is a radical selected from the group consisting of abietyl, hydroabietyl, tetrahydroabietyl, dihydroabietyl, dihydroabietyl, dihydropimaryl, tetrahydropimaryl, borneyl, alpha-terpineyl, B-terpineyl, V-terpineyl, methyl, and dihydroterpineyl, and PA1 R' is a radical selected from the group consisting of abietyl, hydroabietyl, tetrahydroabietyl, dihydroabietyl, dehydroabietyl, dihydropimaryl, tetrahydropimaryl, borneyl, alpha-terpineyl, B-terpineyl, methyl, and dihydroterpineyl. PA1 m and n are each integers of 2 or 3, PA1 R' is an alkylene containing 2 to 12 carbon atoms, PA1 R" is an alkyl containing 1 to 20 carbon atoms, PA1 X is hydrogen or --OC--C.sub.n H.sub.2n SR, at least one of which is --OCC.sub.n H.sub.2 n SR, PA1 the R.sub.1, R' and R" moieties in one compound being the same or different. PA1 n has a value of from 2 to 4; and PA1 Z is an aliphatic hydrocarbon of the formula: EQU C.sub.y H.sub.2y +2-n
As already stated, the properties of ethylene polymers are excellent for practical service conditions. The thermoplastic nature of ethylene polymers, however, represents a deficiency in properties with respect to electrical insulation in certain emergency situations. A conductor overheated as a result of an overload or short circuit can reach temperatures of 160.degree. C. and higher at which insulation consisting of ethylene polymer plastic can melt and flow away from the conductor, leaving areas of bare metal which can cause severe electrical fires and other serious damage. To overcome this problem, ethylene polymers have been subjected to treatments that result in cross-linking the polymer and thereby increase the tensile strength of the polymer both at ambient temperatures and especially at elevated temperatures. The first cross-linking treatments were done by exposure to high energy ionizing radiations as discovered by A. Charlesby (Proceedings of the Royal Society, London, 1952, Vol. 215A, Pages 187-214).
Later, the simpler method of cross-linking initiated by the thermal decomposition of added chemical initiators such as organic peroxides and azo compounds was introduced. The improvement in high temperature mechanical properties obtainable by cross-linking an ethylene polymer was demonstrated by S. Bonotto (Journal of Applied Polymer Science, 1965, Vol. 9, Page 3822) as shown:
TABLE I __________________________________________________________________________ Crosslinked, Unfilled Polyethylene Cross- linking Yield density Ultimate tensile Secant modulus, strength Gel factor c, Melt strength psi PE.sup.Elongation, % psi (23.degree. C.), content, mole/cm..sup.3 Polymer index 23.degree. C. 160.degree. C. 23.degree. C. 160.degree. C. 23.degree. C. 160.degree. C. psi % .times. 10.sup.-3 a __________________________________________________________________________ Low-density PE.sup.b 2 1,800 Flows 600 Flows 20,000 Flows 1,200 -- -- Crosslinked low-density PE.sup.b (3% peroxide.sup.c) 0 2,280 66 500 100 11,000 166 d 88.3 0.16 Crosslinked low-density Pe.sup.b (4.5% peroxide.sup.c) 0 2,360 118 490 100 10,400 237 d 88.3 0.23 Crosslinked low-density PE.sup.b (3% peroxide,.sup.c 10% triallyl cyanurate) 0 2,350 66 193 15 11,350 445 d 93.0 0.43 High-density PE.sup.e 5 3,000 Flows 270 Flows 150,000 Flows 4,600 -- -- Crosslinked high-density PE.sup.e (3% peroxide.sup.c) 0 2,300 245 265 80 55,000 474 2,400 98.0 0.45 Vulcanized rubber.sup.f 0 2,200 170 350 40 705 600 d 87.7 0.57 __________________________________________________________________________ .sup.a See Experimental Section. .sup.b DYNH, density 0.919. .sup.c Dimethyl peroxide: cured at 160.degree. C., 15 min. .sup.d No true yield point. .sup.e DMD-7000, density 0.96. .sup.f SBR formulation: SBR rubber (100), HAF black (50), sulfur (1.33), Altax (1.0) cumate (0.16).
Similarly Bonotto has shown that cross-linking by the action of 1.5% 2-phenyl-2-propyl peroxide (dicumyl peroxide) at 160.degree. C. increases the 23.degree. C. tensile strength of ethylene-ethyl acrylate copolymer from 1910 psi (130 kg per square cm) to 2980 psi (230 kg per square cm) for the crosslinked polymer. More significantly the cross-linked polymer retains 50 psi (3.4 kg per square cm) tensile strength at 160.degree. C. while the unmodified polymer flows at 160.degree. C.
Disclosures of various initiators and methods for the cross-linking of ethylene polymers with the help of chemical additives include the use of peroxides decomposing above the melting point of the polymer, by H. Haeberli in Swiss Pat. No. 475,083 of Aug. 29, 1969; acetylenic bis-peroxycarbonic acid diesters, for example 2,5-dimethyl-2,5-bis(ethoxycarbonylperoxy)-3-hexyne, by O. Mageli in U.S. Pat. No. 3,297,738 of Jan. 10, 1967; acetylenic bis-dialkyl peroxides, such as 2,5-bis(t-butylperoxy)-2,5-dimethyl-3-hexyne, by H. Blanchard in U.S. Pat. No. 3,334,080 of Aug. 1, 1967; the same hexyne as well as the saturated analog 2,5-bis(t-butylperoxy)-2,5-dimethylhexane by M. Narkis in Journal of Applied Polymer Science 1969, Vol. 13, Pages 713-720, and the latter two as well as bis(1,3-di-t-butylperoxyisopropyl)benzene by Dr. Braun in Chemical Abstracts 1971, Vol. 74, 64508 g.
Unfortunately, the improvement in mechanical properties of ethylene polymers achieved through cross-linking has much increased the difficulty of safeguarding the properties of the polymer once its fabrication and shaping into end products such as electrical insulation is completed, in order to prevent premature deterioration of the product. Use of stabilizing additives and antioxidants as practiced with ethylene polymers that are not subjected to cross-linking is ineffective or inapplicable in the case of the cross-linked polymers. As summarized by Nass, stabilizers are effective because they decompose peroxides that form as a result of antioxidation or because they interrupt the propagation of chain reactions involving free radicals. This means that the customary stabilizers can and do destroy peroxides used as initiators of cross-linking and are themselves destroyed as well. Thus the search for a stabilizing additive that can be compounded with an ethylene polymer that is cross-linked chemically and protect the properties thereof is a problem entirely different from that of stabilizing thermoplastic ethylene polymers. For example, D. Simunkova et al (Chemical Abstracts 1970, vol. 73, No. 88573c) reported on the dicumyl peroxide initiated cross-linking of polyethylene in the presence of the antioxidants 2,2'-methylene bis(4-methyl-6-t-butylphenol), dilauryl thiodipropionate, zinc mercaptobenzimidazole, and phenylbeta-napthylamine. The authors observed that interaction of antioxidants with free radicals lowered both the efficiency of cross-linking and the oxidation stability of the polymer.
A. Bluestein, et al (Chemical Abstracts, 1968, Vol. 68, No. 30555f) reported on the effectiveness of polymerized trimethyldihydroquinoline as a high temperature antioxidant for cross-linked polyethylene. The authors note that many different forms of the antioxidant are available and results obtained vary depending on which form is used.
R. Catte in German Offenlegungschrift No. 2,062,603 of June 24, 1971 (see Chemical Abstracts 1971, vol. 75, No. 110816b) disclosed a synergistic antioxidant combination of a phenol for example 2,6-di-t-butyl-4-methylphenol, with a 1,2-dihydroquinoline and an organic zinc salt, for example zinc mercaptobenzothiazole, zinc dimethyl-dithiocarbamate, and zinc mercaptobenzimidazole.
General Electric Co. in British Pat. No. 871,284 of June 28, 1961 disclosed a curable polyethylene containing dicumylperoxide and 1,2-dihydro-2,2,4-trimethylquinoline as antioxidant.
M. Watanabe et al in Japan Publication No. 10516/64 of June 13, 1964 disclosed cross-linking polyethylene composition with an organic peroxide as cross-linking agent, an antioxidant and 4,4'-thio-bis(6-t-butyl-m-cresol) as radical acceptor.
K. Tezika et al in Japan Publication No. 18461/74 of May 10, 1974 disclosed cross-linking polyethylene composition by incorporating 4,4'-thiobis(6-t-butyl-3-methyl phenol), sulfur and di(ortho-benzamide phenyl) disulfide into polyethylene containing an organic peroxide.
A. DiBattista in U.S. Pat. No. 3,763,095 of Oct. 2, 1973 disclosed stabilized cross-linking polyethylene with a bisphenol sulfide having the formula (1): ##STR1## In addition to the large number of thiodipropionate esters disclosed as ethylene polymer stabilizers since the pioneer disclosure of M. Gribbins, a variety of esters containing the thioalkylenecarboxylic acid ester structure have been disclosed. Among these, H. Braus in U.S. Pat. Nos. 3,504,012 of Mar. 31 and 3,538,047 of Nov. 3, 1970 disclosed 2-hydroxyethyl 3,5-di-t-butyl-4-hydroxybenzylthiopropionate. H. Eggensperger in U.S. Pat. No. 3,832,838 of Aug. 27, 1974 disclosed hydroxydialkylbenzylthioalkanecarboxylic acid ester having the formulae: ##STR2## wherein n is an integer from 1 to 4,
M. Minagawa in Japan Kokai 101435/73 of Dec. 20, 1973 disclosed polymer stabilizers having the formula R--O--A--O--R, in which there can be one or more --A-- group and A can be a residue of a dicarboxylic acid, a polyhydric phenol or a polyhydric alcohol; R and R' , can be alkyl or R'CO where R'CO is a residue of a monocarboxylic acid. The only monocarboxylic acids shown are stearic acid, salicylic acid, and 3(3,5-di-t-butyl-4-hydroxyphenyl)propionic acid.
H. Schutze in U.S. Pat. No. 3,630,991 of Dec. 28, 1971 disclosed non-exuding and non-volatile sulfur containing esters of cyclic terpene alcohols for the stabilization of 2 to 8 carbon alpha-olefin polymers together with hindered phenols. Schutze's esters may be represented by the structural formulae EQU ROOC CH.sub.2 (CH).sub.n SR.sub.a EQU ROOC CH.sub.2 (CH.sub.2).sub.n SS (CH.sub.2).sub.n CH.sub.2 COOR' ##STR4## EQU ROOC CH.sub.2 (CH.sub.2).sub.n S(CH.sub.2).sub.n S(CH.sub.2).sub.n CH.sub.2 COOR' EQU ROOC CH.sub.2 (CH.sub.2).sub.n S(CH.sub.2).sub.n S(CH.sub.2).sub.m CH.sub.3
where
A. Onishi, in U.S. Pat. No. 3,629,194 of Dec. 21, 1971 disclosed a polyolefin resin stabilized against thermal aging with esters of alkyl thiopropionic or alkyl thiobutyric acid with a polyol having up to five hydroxyl groups, in combination (optionally) with a phenolic antioxidant. The alkyl thiopropionic or alkyl thiobutyric acid esters are defined as having one of the formulae: ##STR5## wherein R is an alkyl of 8 to 30 carbon atoms,
The phenolic antioxidants are defined by Onishi as mono- or polyhydric phenolic compounds in which at least one of the ortho positions to a hydroxyl group is substituted by an alkyl, aralkyl, or cycloalkyl group.
The substituents preferably contain carbon atoms of a number of the order of 3 to 10, and the alkyl group, atoms inclusive of that in an aralkyl and cycloalkyl groups can be unsaturated. The phenolic compounds may be further substituted, and the phenolic compounds may be polyphenolic compounds such as bisphenolic, trisphenolic, or tetrakisphenolic compounds in which phenolic nuclei are connected by a connecting group such as an alkylene, a thioether, or a triazinoxyl group.
M. Dexter in U.S. Pat. No. 3,758,549 of Sept. 11, 1973 disclosed alkyl esters derived from alkyl thioalkanoic acids and alkane polyols, such as pentaerythritol tetrakis, 3-n-dodecylthiopropionate, and ethylene-bis-3-n-dodecylthiopropionate. These are used in combination with phenolic antioxidants to effectively stabilize polyolefins from the deleterious effects of heat and oxygen. The alkyl esters are defined by the formula: ##STR6## wherein R is an alkyl group of from one to eighteen carbon atoms,
in which y has a value of from 2 to 18 when n is 2 and a value of from 3 to 6 when n is greater than 2, the value of y in all cases being equal to or greater than that of n.
M. Minagawa in Japanese Kokai No. 75/106881 of Aug. 27, 1975 disclosed stabilized resin compositions containing 3-alkylthiopropionate esters of alcohols containing a nitrogen-heterocyclic ring, for example tris(2-hydroxyethyl isocyanurate) and optionally a phenolic antioxidant.